1. Field of the Invention
The present invention relates to an altitude decision system for an internal combustion engine and to an engine operating parameter control system using the altitude decision system. In particular, the invention is useful for a system capable of achieving a fuel injection rate, an intake air flow and ignition timing which is optimized for the altitude of the engine.
2. Description of the related art
In the prior art, as disclosed in Japanese Patent Laid-Open No. 8339/1989, there is prepared an altitude map, in which an altitude corresponding to an intake air flow (Q.sub.a) for both a predetermined angle of opening of a throttle valve and a predetermined number of revolutions of the engine is predetermined and stored in the form of a map in a memory. The altitude is determined from the aforementioned memory map using the intake air flow (which is measured by an air flow meter for a predetermined throttle valve opening (.theta..sub.TH) detected by a throttle sensor) and the predetermined engine number of revolutions (N.sub.e) (detected by a revolution number sensor). A plurality of predetermined maps of .theta..sub.TH and N.sub.e are required for different intake air flow quantities Q.sub.a. To avoid overloading memory storage and to reduce software computations, the number of memory maps is restricted to, say, 100 m increments in height from sea level.
In the system of the forementioned prior art, if a mass air flow sensor is used, the performance in the steady state is not affected even at a high altitude by an over rich air/fuel (A/F) ratio in a partial region of operation unlike the system using a capacity type air flow sensor. If the vehicle goes up to a high altitude with the control constants set for a low altitude, various difficulties occur due to the air density dropping.
In order to start the engine, for example, the startability will be deteriorated due to shortage of the intake air flow unless the open duty of the idle speed control (ISC) valve is made larger than that for low altitude. Unless the fuel injection pulse width at the start is reduced, on the other hand, there arises a problem in that the A/F ratio becomes over rich to deteriorate the startability. For acceleration, moreover, the ability to accelerate will be deteriorated by the rich A/F ratio unless the injection rate is reduced. Unless the ignition timing is retarded, moreover, there will arise another problem in that the engine knocks when the throttle valve is fully opened.
The foregoing problems relate to the engine performance. Furthermore, because of the requirement for an altitude decision map, problems such as an increase in the burden upon the software occur which is also adversely affected with regard to accuracy due to variations in performance of the throttle sensor and the air flow sensor.
An object of the present invention is to provide an altitude decision system for an internal combustion engine and an engine operating parameter control system using the same which is free of any increase in the burden upon the software and which is able, even at a high altitude, to achieve the same performance of the vehicle as at low altitude.